doc/bdev: add entry for the delay bdev module.

`rpc.py delete_crypto_bdev CryNvmeA`

# Delay Bdev Module {#bdev_config_delay}

The delay vbdev module is intended to apply a predetermined additional latency on top of a lower

level bdev. This enables the simulation of the latency characteristics of a device during the functional

or scalability testing of an SPDK application. For example, to simulate the effect of drive latency when

processing I/Os, one could configure a NULL bdev with a delay bdev on top of it.

The delay bdev module is not intended to provide a high fidelity replication of a specific NVMe drive's latency,

instead it's main purpose is to provide a "big picture" understanding of how a generic latency affects a given

application.

A delay bdev is created using the `bdev_delay_create` RPC. This rpc takes 6 arguments, one for the name

of the delay bdev and one for the name of the base bdev. The remaining four arguments represent the following

latency values: average read latency, average write latency, p99 read latency, and p99 write latency.

Within the context of the delay bdev p99 latency means that one percent of the I/O will be delayed by at

least by the value of the p99 latency before being completed to the upper level protocol. All of the latency values

are measured in microseconds.

Example command:

`rpc.py bdev_delay_create -b Null0 -d delay0 -r 10 --nine-nine-read-latency 50 -w 30 --nine-nine-write-latency 90`

This command will create a delay bdev with average read and write latencies of 10 and 30 microseconds and p99 read

and write latencies of 50 and 90 microseconds respectively.

A delay bdev can be deleted using the `bdev_delay_delete` RPC

Example command:

`rpc.py bdev_delay_delete delay0`

# GPT (GUID Partition Table) {#bdev_config_gpt}

The GPT virtual bdev driver is enabled by default and does not require any configuration.